The heart is the central element of the cardiovascular system and includes four chambers known as the left and right atria and left and right ventricles. Venous blood from the body is received by the right atrium and passed to the right ventricle, where it is pumped to the lungs via the pulmonary artery. Once oxygenated, this blood returns to the left atrium and is pumped to the aorta, by contraction of the left ventricle, for circulation throughout the body.
The operation of the heart is the result of muscular contractions of the chambers induced by electrical impulses. A sinoatrial node is the normal pacemaker of the heart and produces on the order of 50 to 80 of these electrical impulses per minute. Each impulse travels first across the atria and then across the ventricles, causing the atrial and ventricular muscle fibers to sequentially contract. After contraction, these muscle fibers repolarize and return to a resting state in preparation for the next electrical impulse.
Proper operation of the cardiovascular system is thus dependent upon the operation of the sinoatrial node. In certain instances, however, the sinoatrial node continues to operate properly but the heart experiences an arrhythmia or irregularity that affects the ability of the muscle fiber to properly respond to the impulses produced by the sinoatrial node. The most serious form of arrhythmia is known as "ventricular fibrillation."
Ventricular fibrillation is a potentially fatal condition caused by uncoordinated electrical impulses that produce twitching of individual ventricular muscle fibers with little or no contraction of the ventricles as a whole. The heart is, thus, unable to transport blood and, with it, oxygen to the various parts of the patient's body. Ventricular fibrillation may be induced by physiological phenomena, such as coronary disease and myocardial infraction, as well as environmental phenomena, including electric shock, drug toxocity, and drowing. The heart is particularly susceptible to ventricular fibrillation during repolarization of the ventricular muscle fiber.
Defibrillation involves the delivery of electric energy to the heart, either directly through an open chest or indirectly through the chest wall, to terminate ventricular fibrillation. The defibrillation energy must be sufficient to depolarize a relatively large section of the muscle fiber and, once removed, allow the sinoatrial node to resume pacing of the heart. The energy required to accomplish this depends on a number of factors including, for example, the size of the patient.
Although distinguished in severity from ventricular fibrillation, a number of other arrhythmias may be experienced by the heart. For example, atrial fibrillation and atrial and junctional tachycardias may occur, disrupting proper operation of the heart. Because some blood is still pumped by the heart, however, these conditions are not as serious as ventricular fibrillation. In fact, the application of electric energy to the heart to terminate these conditions may even be elective. In any event, however, energy should not be applied to the heart during the vulnerable period of ventricular repolarization, or the more severe condition of ventricular fibrillation may be induced.
The termination of irregularities other than ventricular fibrillation is commonly referred to as synchronized cardioversion, rather than defibrillation. Synchronized cardioversion typically involves the application of a lower level of energy to the heart during a select portion of the ECG waveform other than the vulnerable period of ventricular repolarization. For the purposes of this application, however, the term "defibrillation" will be understood to include the application of energy to the heart to terminate ventricular fibrillation, as well as other arrhythmia and irregularities traditionally considered to be the subject of synchronized cardioversion.
Addressing the prior art of defibrillation, conventional defibrillators store energy for discharge, for example, through a pair of paddle electrodes positioned on the patient's chest. Upon discharge, a brief pulse of energy is applied to the patient's heart, repolarizing the heart's muscle fiber and allowing the sinoatrial node to reinitiate proper pacing in the manner described above. Typically, the operator is free to select from any one of a plurality of energy discharge levels, depending upon a number of characteristics, including, for example, the size of the patient and the type of irregularity experienced by the heart.
As will be appreciated, proper control of the level of energy applied to the patient's heart is important to successful defibrillation. For example, if the energy discharged by the defibrillator is less than that desired by the operator, the discharge may be unsuccessful in terminating the irregular operation of the heart. Although a subsequent discharge at a higher energy level can still be employed, even momentary delays in the restoration of proper circulation can significantly increase the damage suffered by certain of the patient's organs, such as the brain. The patient may also be injured if the energy discharged by the defibrillator is greater than expected. In that regard, an extremely high discharge may damage the muscle fiber of the heart, impairing its operation. In addition, an unexpectedly high discharge applied to correct a mild arrhythmia or irregularity might possibly induce a more serious, ventricular fibrillation.
Several approaches have been adopted in the past to control defibrillation discharge levels. For example, in U.S. Pat. Nos. 3,860,009 (Bell et al.), 3,862,636 (Bell et al.), and 3,886,950 (Ukkestad et al.), defibrillators are disclosed in which the operator selects the desired level of energy to be discharged to the patient. Basically, these defibrillators employ information concerning the voltage and current applied to the patient by a storage capacitor, along with the interval of time that they applied, to compute the energy transferred. In that regard, the conventional unit of energy is the joule, with one joule being equal to the energy provided by a one-volt potential and one-ampere current for one second. Thus, by measuring the voltage and current and controlling the duration of their application to the patient, the defibrillators disclosed in these patents are able to control the energy transferred to the patient.
This general approach, however, relies upon information about the voltage or current that may be somewhat inaccurate. More particularly, the accuracy of the voltage, current, and time measurements may vary from one defibrillator to the next or for the same defibrillator over an interval of time. Thus, although such prior art defibrillators do attempt to control discharges of energy, they do not address the need for defibrillator calibration to ensure that the computed energy discharge actually corresponds to the desired energy discharge.
In another prior art arrangement, an energy level selection switch is employed to selectively allow different resistors to be switched into a voltage divider formed with a first variable resistor placed across an energy storage capacitor. By selectively switching a particular resistance into the divider, the voltage applied to the capacitor and, hence, the energy stored for discharge, can be controlled. The variable resistor can also be adjusted to proportionally increase or decrease the voltage applied to, and energy stored by, the capacitor at each energy level.
A plurality of displays are also employed by this arrangement to indicate the particular energy level selected. More particularly, another voltage divider is employed to produce a plurality of reference voltages for use by comparators in controlling the operation of the displays. These comparators compare the voltage applied to the capacitor with one or more reference voltages to ensure that the applied voltage is within the range required to provide the desired energy storage and discharge. A second variable resistor common to each leg of this voltage divider allows the reference voltages input to the comparators to be adjusted so that the relationship between the capacitor voltages and the activation of a particular display can be controlled.
Although the adjustment of either of the variable resistors noted above represents a calibration, some improvement can still be made in the calibration of the defibrillator. For example, it would be desirable if calibration of the defibrillation discharge could be accomplished separately for each discharge level, rather than via a single adjustment that affects each level simultaneously. Such an approach would avoid the necessity of repeating the calibration performed for previous levels when an adjustment is made in a subsequent energy level calibration. It would also be desirable if the calibration procedure could be performed on the basis of the energy actually discharged rather than on parameters such as the voltage applied to the capacitor to develop a stored charge.